"But we are only just beginning to understand how the brain as an organ is influenced by the nutrients it derives from the foods we eat and how diets have an impact on our mental health."

And he added that addressing mental health problems with changes in diet was showing better results in some cases than using drugs or counselling.

The report, Feeding Minds, pointed out the delicate balance of minerals, vitamins and essential fats consumed had changed in the past five decades.

Researchers said the proliferation of industrialised farming had introduced pesticides and altered the body fat composition of animals due to the diet they are now fed.

For example, the report said chickens reach their slaughter weight twice as fast as they did 30 years ago, increasing the fat content from 2% to 22%.

The diet has also altered the balance of vital fatty acids omega-3 and omega-6 in chickens which the brain needs to ensure it functions properly.

Fats

In contrast, saturated fats, consumption of which has been increasing with the boom in ready meals, act to slow down the brain's working process.

The report said people were eating 34% less vegetables and two-thirds less fish - the main source of omega-3 fatty acids - than they were 50 years ago.

Such changes, the study said, could be linked to depression, schizophrenia, attention deficit hyperactivity disorder (ADHD) and Alzheimer's disease.

The two groups urged people to adopt healthier diets, with more fresh vegetables, fruit and fish, and called on the government to raise awareness about the issue.

Report researcher Courtney Van de Weyer said: "The good news is that the diet for a healthy mind is the same as the diet for a healthy body.

"The bad news is that, unless there is a radical overhaul of food and farming policies there won't be healthy and nutritious foods available in the future for people to eat."

Rebecca Foster, a nutrition scientist at the British Nutrition Foundation, said: "The evidence associating mental health and nutrient intake is in its infancy, this is a very difficult association to research and in many cases results are subjective.

"Therefore, it is difficult to draw conclusions about the association between mental illness and dietary intake at this point.

"However, the nutrient recommendations outlined in this report are in line with recommendations for good health, which should continue to be advocated by all health professionals."